If you open the eCart Object, you will first need to modify the Total Price Calculation. Click on the Calculations Tab and select the Total Price Calculation and modify it to to not include the tax by adding (1/1.1).
And then you will need to add a tax rule. Click on the Tax tab and click the + button. type the name of the tax rule and click the + button. You should select 'total number of unique items in the cart' for your condition and make sure to select > than 0. For the calculation you should select 'based on the cart sub total' and then multiply it by 0.1.